A while back, I mentioned receiving a Springfield Armory XD-S 9 mm 4.0 here at the 'desk. While it does come with both a seven-round and a nine-round magazine, more is always better, right?
Shipping with both X-Tension backstraps, the factory magazines can be configured to fit the XD-S with either of the two backstraps that accompany the pistol. Whether a slimmer grip is preferred or the original, the X-tension magazines will match the profile.
With the X-tension magazine inserted in the XD-S, it turns the shortened grip into a full-size, allowing all three fingers to acquire purchase. Two additional rounds fit into the longer magazine, making the XD-S in 9 mm a 10-round-capacity arm, while still maintaining the ultraslim width. Since grip size is more often the limiting factor in how concealable your firearm can be, having the flush magazine loaded with an extended magazine (or two) as backup makes for a combination that offers plenty of firepower in a package simple to keep under wraps.